warn for repeated queries in the same scope, suggest caching selector results

[jokeyrhyme]

Don't:

$('button').prop('disable', false);
$('button').css('background-color', 'red');

Do:

var button$ = $('button');
button$.prop('disable', false);
button$.css('background-color', 'red');

Avoiding repeat queries is good for performance / UI-responsiveness.

[jokeyrhyme]

It's difficult to make this an error, as there are potentially legitimate uses for repeat queries.